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Mono-piles being installed at a challenging site in Gothenburg harbour are designed to snap off.

Damon Schünmann finds out why.

When it designed the piles to support lighting beacons in Sweden's Gothenburg harbour, Danish contractor MTHojgaard (MTH) had an important consideration; they must be able to break off just above the seabed.This may seem contradictory, but for the ships that use the harbour channel it is a critical point.

The beacons are to guide shipping through the two safe routes to the Baltic Sea and form the SKr135M (£10M) design and build contract awarded by the Port of Gothenburg and the Swedish Maritime Safety Inspectorate (Sjofartsverket).

But mindful of errant shipping, Danish designer COWI built in a weak link in the structure 1m above the harbour bed that will break if hit by a large vessel, but will resist the impact from the regular winter ice that collects at the entrance to the harbour.

The site presents some tough challenges.

Subcontractor Seacore won the £2.7M contract to install the 34 mono piles and is facing a combination of severely undulating ground and hard rock that is difficult to drill.

The bedrock is granite with quartz veins and gneiss overlaid in places with soft silty clay and sand. Progress is marked by sudden and extreme changes between the overburden and bedrock from very soft to extremely hard.

Drilling supervisor Martin Empson says:

'We're drilling through the hardest rock at an average rate of just over 0.1m/h which through such unfractured and un-weathered rock is not a bad rate of penetration.'

But it is not all about raw drilling power. The rig crew decided to remove one of the four motors that drive the power swivel to increase its rotary speed and at times the drilling has been a tentative process. Getting purchase on hard materials on steep slopes has seen entire shifts spent 'tickling' the bedrock to establish a socket.

'At the entrance to the southern channel we were about 40m from an outcrop of rock and the water was about 20m deep which shows the types of incline we have been working with, ' says Empson.

To prevent the drill slipping down the slope when beginning a socket, Seacore is using a double gripping mechanism to secure it in position - a system that it claims is unique.

'The double gripper gates are the biggest reason that the job has gone so well, ' says project manager Phil Wilkinson. The job that started a week late on 9 March is now three weeks ahead of schedule and is due to complete in early autumn.

Helping MTH to plot the drilling positions in the varying bedrock levels are the results of a sonar site investigation to confirm earlier borehole findings. .

Swedish marine survey subcontractor Marin Mätteknik (MMT) took readings at 5m centres in grids of five readings by five. Efforts were sometimes hampered by sound waves being disrupted by gas pockets in the softer material, or reflecting against each other because of the sloping surfaces of the rock profile, but it was accurate enough to produce three-dimensional picture of the undulating contours of the harbour bed.

MTH site manager Lars Forsberg explains that this has proved particularly useful in cases where a drilling position might need to be adjusted and the predictions can now be made about a nearby site of similar depth.

Seacore project manager Phil Wilkinson says: 'Because the weak link has to be 1m above bed level, the piles have been individually designed. If a survey is good then designers can economise on pile design and contractors can hone their installation techniques making the project more efficient.

Seacore has installed three types of piles to cater for the differing ground conditions. The nine 'type ones' are being rock socketed into locations where there is little or no overburden.

The barge crew drill a 1.7m diameter hole to depths between 6.4m to 8.2m and then locate a 1.7m sacrificial casing shoe into the top of the socket to keep the hole open and clean.

Next they float a 1.5m diameter watertight pile into position above the socket and flood it so that they can locate it into the casing shoe.

This leaves a 200mm annulus between pile and shoe which the rig crews fill with a special grout suitable for the low water temperatures.

Empson says: 'Because we are working in water that is extremely cold, we went to Densit [a Danish company] and they supplied a grout [Ducorit S1] that will cure in colder waters and it comes out to a much higher MPa' He continues: 'The client asked for 10Mpa, but after a 12 hour cure the grout is reaching 20MPa and the final cure strength is much higher. We could have waited days with a normal grout which would be very expensive.'

Where there is a silt overburden the rig crew are installing wider 1.8m Type two units in a drive-drill-drive sequence. After positioning using a global positioning satellite system, a vibro-hammer drives the casing until it founds on bedrock. The drillers then remove the hammer and install the drill rig and string inside the pile casing and drill to depth.

They then engage an under-reamer that extends out to drill to the same diameter as the outside diameter of the pile. Once under-reaming is completed up to the toe of the pile, the pile is once more vibro-hammered into the socket to final depth.

Type three piles come into play at locations where there is only the silty clay and sand with no underlying rock. Although a vibro-hammer is sometimes used to drive to depth, many of the type threes can be pushed to depth using the hydraulic gate gripper system.

At some of the type two and three piles sites friction-wheels have been needed. Seventeen of the large doughnut shaped 8t frames have been lifted over the pile and placed on the seabed to add support and increase lateral stability. They are filled with rock to give a total weight of 200t, and act like an anchor in the softer areas of the seabed.

Meanwhile Dutch contractor Boskalis has been widening the channels in which the beacons will sit. It has dredged 11M. m 3of clay and removed 0.5M. m 3of rock using drill and blast.

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